Many beginning Perl programmers don’t understand the map and grep functions and avoid them. But they’re the key to many solutions under the heading of “list processing” and once you become familiar, you’ll start to see lots of opportunities to use them. Let’s demystify these important functions!

1. DON’T FEAR MAP & GREP
LIST PROCESSING FOR FUN AND PROFIT

2. MARK GARDNER 22 JUNE 2022
WHY USE map AND grep?
Simplifies your code when processing lists
and arrays
Replace or augment for and foreach loops to
better describe what’s going on
Stepping stone to other list processing
functions from CPAN
From this:
my @greetings;
foreach my $name (@people) {
push @greetings, "Hello $name";
}
To this:
my @greetings = map {"Hello $_"} @people;

3. MARK GARDNER 22 JUNE 2022
map IS THE LIST TRANSFORMER
Two inputs: an expression or block of code,
and a list to run it on
Expression style:
my @foo = map bar($_), @list;
Block style:
my @foo = map { bar($_) } @list;
Block is better—easier to to read, more
flexible when building complicated lists
Another example, building a hash:
my %IS_EXIT_WORD = map { $_ => 1 }
qw(q quit bye exit stop done last finish);
...
die if $IS_EXIT_WORD{$command};

4. MARK GARDNER 22 JUNE 2022
grep IS THE LIST FILTER
Inspired by Unix grep command, but not just
for regular expressions
Simple example:
my @months_with_a = grep { /[Aa]/ } qw(
January February March
April May June
July August September
October November December
);
Running functions that return true/false:
my @results =
grep { large_database_query($_) } @foo;
Check for defined-ness:
say $_ > 5 ? "$_ is bigger"
: "$_ is equal or smaller"
for grep { defined } @numbers;

5. MARK GARDNER 22 JUNE 2022
THIS MACHINE KILLS ARRAY ELEMENTS
BEWARE SIDE EFFECTS
What does this do?
my @needs_docs =
grep { s/.pm$/.pod/ && !-e } @pm_files;
Should have used a variable or refactored:
my @needs_docs = grep { no_docs($_) }
@pm_files;
sub no_docs {
my $file = shift;
$file =~ s/.pm$/.pod/;
return !-e $file;
}
Or non-destructive substitution:
use v5.14;
my @needs_docs = grep { !-e s/.pm$/.pod/r }
@pm_files;
foreach if you really want side effects

6. MORE ON LISTS:
phoenixtrap.com/tag/lists